[gtk/fix-icontheme-invalidation] icon-theme: Fix deadlock in GktIconTheme dispose

[Alexander Larsson <alexl src gnome org>]

commit 1e71f7f0ea55eadaeee1728437806e71fce70eac
Author: Alexander Larsson <alexl redhat com>
Date:   Wed Apr 22 14:20:33 2020 +0200

    icon-theme: Fix deadlock in GktIconTheme dispose
    
    When freeing the display GtkIconTheme and that was the last owner we
    ran into a deadlock, because we unref the "next-to-last" ref inside a
    gtk_icon_theme_ref_aquire/release() pair, which makes the final unref
    to happen in the release(), while the ref lock still was held.
    The unref triggers dispose which tries to NULL out the ref, but that then
    deadlocks on the mutex being held by the caller already.
    
    We fix this by moving the release unref outside the lock. This is safe
    because refcounts are atomic, and we *do* own the ref.

 gtk/gtkicontheme.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkicontheme.c b/gtk/gtkicontheme.c
index bf8bee3eed..ffc0688360 100644
--- a/gtk/gtkicontheme.c
+++ b/gtk/gtkicontheme.c
@@ -551,9 +551,16 @@ gtk_icon_theme_ref_aquire (GtkIconThemeRef *ref)
 static void
 gtk_icon_theme_ref_release (GtkIconThemeRef *ref)
 {
-  if (ref->theme)
-    g_object_unref (ref->theme);
+  GtkIconTheme *theme;
+
+  /* Get a pointer to the theme, becuse when we unlock it could become NULLed by dispose, this pointer still 
owns a ref */
+  theme = ref->theme;
   g_mutex_unlock (&ref->lock);
+
+  /* Then unref outside the lock, because otherwis if this is the last ref the dispose handler would 
deadlock trying to NULL ref->theme */
+  if (theme)
+    g_object_unref (theme);
+
 }
 
 static void